There is known a hydraulic drive system for use in civil engineering and construction machines such as hydraulic excavators, typically called a load sensing system, wherein the delivery flow rate of a hydraulic pump, i.e., the pump delivery rate, is controlled so as to hold the delivery pressure of the hydraulic pump, i.e., the pump pressure, higher by a fixed value than the load pressure of an actuator, thus causing the hydraulic pump to deliver a hydraulic fluid only at the flow rate necessary for operation of the actuator. As disclosed in JP, A, 60-11706, for example, the load sensing system includes a pump regulator for load sensing control (LS control), which comprises an actuator cylinder for controlling the displacement volume of the hydraulic pump, and a control valve operated responsive to the differential pressure between the pump pressure and the load pressure for controlling operation of the actuator cylinder. The control valve is provided with a spring for urging the control valve in a direction opposite to the differential pressure between the pump pressure and the load pressure. The control valve is operated so as to keep the force of the spring in balance with the differential pressure between the pump pressure and the load pressure. The pump delivery rate is thereby controlled such that the above differential pressure is held at a fixed value corresponding to the spring force, i.e., a target differential pressure.
Furthermore, the load sensing system generally has a pressure compensating valve disposed upstream of a flow control valve to control the differential pressure across the flow control valve, thereby ensuring a flow control function to cope with fluctuations in the differential pressure between the pump pressure and the load pressure.
The pressure compensating valve generally comprises a valve spool slidably disposed in a valve housing and having a flow control section which serves as a variable restrictor, and first and second control chambers formed in the valve housing in facing relation to each other and accommodating the opposite ends of the valve spool respectively. The load pressure of the actuator (the outlet pressure of the flow control valve) is introduced to the first control chamber for urging the valve spool in the valve-opening direction, and the inlet pressure of the flow control valve is introduced to the second control chamber for urging the valve spool in the valve-closing direction. A spring for urging the valve spool in the valve-opening direction is disposed in the first control chamber to provide a target value for the pressure compensation.
When the differential pressure between the inlet pressure of the flow control valve and the load pressure of the actuator respectively introduced to the first and second control chambers, i.e., the differential pressure across the flow control valve, becomes larger than the setting value of the spring, the valve spool is caused to move in the valve-closing direction so that the differential pressure across the flow control valve is controlled to be held at the setting value of the spring, i.e., the target pressure. As a result of the differential pressure across the flow control valve being controlled in this way, the flow rate of hydraulic fluid passing through the flow control valve, i.e., the flow rate of hydraulic fluid supplied to the actuator, is adjusted to a value proportional to the opening area of the flow control valve, thus permitting stable control of the actuator.
One pressure compensating valve of this type is disclosed in U.S. Pat. No. 4,688,600, for example.
However, the hydraulic drive system equipped with the above-mentioned conventional pressure compensating valve has accompanying problem as follows.
In the case where the civil engineering and construction machine is a hydraulic excavator and the actuator is a boom cylinder for driving a boom as one component of a front mechanism, for example, when the flow control valve is quickly operated to change a drive speed of the boom cylinder during operation of the boom cylinder, the hydraulic fluid being subjected to inertia of the boom serves as a spring and produces a vibration. Once produced, the vibration will not damp or cease soon because the damping capability of the actuator is very poor in a hydraulic system constituted by the conventional hydraulic drive system. Therefore, control accuracy of the boom cylinder is lowered, which tends to a difficulty in realizing the operation as intended by an operator.
An object of the present invention is to provide a hydraulic drive system for civil engineering and construction machines and a pressure compensating valve for use in the system, in which the pressure compensating valve is improved to enhance the damping capability of an actuator and increase the control accuracy of the actuator.